This invention relates generally to radio frequency circuits and more particularly to radio frequency interdigitated coupler circuits.
As is known in the art, an ideal hybrid junction circuit is a four-port network in which a signal incident on any one of the ports divides between two output ports with the remaining port being isolated. The assumption is that all output ports are terminated in a perfect match. Under these conditions, the input to any port is perfectly matched.
Hybrid circuits are frequently used as components in radio frequency systems or subsystems, such as balanced mixers, balanced amplifiers, attenuators, measurement bridges, phase shifters, and modulators. Hybrid circuits are generally classified according to the phase shift between the two outputs. Two basic types of hybrid circuits are 180.degree. hybrids and 90.degree. (quadrature) hybrids.
The increasing use of microwave integrated circuits has produced a need for small, low insertion loss networks which can be easily fabricated using a planar propagation media such as microstrip.
One problem encountered by circuit designers, with regard to hybrid couplers, is the design of couplers having relatively tight coupling (i.e., coupling values less than 6 decibels). As is known in the art, tight coupling in directional couplers for microwave integrated circuits may be achieved by using broadside coupling, reentrant sections, tandem sections, or branch-line couplers. However, couplers using broadside coupling o reentrant sections require multilayer circuitry which is generally difficult to fabricate on ceramic microstrip or monolithic substrates. Couplers using tandem sections or branch-line couplers generally have narrower bandwidths and require much larger substrate areas than single-section coupled line arrangements.
One hybrid coupler having broadband performance relative to the aforementioned versions and attractive for use in planar applications is the interdigitated coupler. An interdigitated coupler, often called a "Lange" coupler, has three or more parallel transmission lines with alternate lines tied together. This configuration provides a hybrid circuit eminently suited for thin-film microwave integrated circuits (MIC) or monolithic circuits. However, one problem with interdigitated Lange couplers is that their use is generally limited to bandwidths of approximately one octave.
Wideband multi-octave directional couplers have been developed using a design approach based on the analogy between multisection directional couplers and stepped impedance transformers. Both 90.degree. and 180.degree. couplers developed with this approach typically provide a significant improvement in bandwidth performance. When such couplers are to be provided in a planar media, stripline is generally the propagation media of choice. As is also known, stripline transmission lines include at least one strip conductor dielectrically spaced between a pair of ground plane conductors. When two strip conductors are used, the strip conductors are overlaid and disposed on separate planes and separated by a dielectric. The use of stripline eliminates problems related to the normally thin spacing required for providing tight coupling between adjacent strip conductors of microstrip transmission lines, such as those described above in the interdigitated Lange coupler. For this reason, overlaid stripline transmission lines generally permit tighter coupling over broader ranges of frequency than microstrip.
On the other hand, because stripline is enclosed by ground planes both below and above the strip conductors, it is generally more difficult to make minor adjustments to the circuit or to attach discrete devices to the circuit. Microstrip is more convenient for these applications since the top surface of the circuit is accessible.
Monolithic microwave integrated circuits (MMIC) or glass microwave integrated circuits (GMIC), similarly have both their active devices and passive circuitry disposed on a top surface of a substrate. Accordingly, microstrip is often selected for the development of circuits which are eventually desired to be fabricated in monolithic microwave integrated circuit form.
It is apparent from the above that both microstrip interdigitated Lange couplers and stripline multi-octave couplers possess advantages and disadvantages not inherent in the other. Accordingly, there is a need for a directional coupler having relatively tight coupling and multi-octave bandwidth characteristics which can be fabricated using microstrip transmission media. Such a directional coupler may be adapted to be used in monolithic or glass microwave integrated circuit applications.